Recently, device applications utilizing magnetoresistance effect and spin-transfer torque have been under development. Spin-transfer torque is a magnetic torque acting on a local magnetic moment in a ferromagnetic material when angular momentum is transferred from conduction electrons to localized electrons along with passage of the spin-polarized current through a ferromagnetic material.
For example, for the application of a magnetoresistive element to an oscillator element, the magnetization of a magnetization free layer needs to reach a spontaneous oscillation state. To reach such a state, required are magnetization reversal induced by a spin-transfer torque generated by a DC bias, and an effective magnetic field in such a direction that the magnetization reversal is blocked. With the actions of the spin-transfer torque and the torque of the effective magnetic field competing against each other, the magnetic moment of the magnetization free layer reaches a state of continuing steady precession (spontaneous oscillation state). In the spontaneous oscillation state, the resistance value is periodically changed, and high-frequency signals are generated at both ends of the magnetoresistive element. Since the precession of magnetization is very fast, high-frequency signals having a frequency of several GHz to several tens of GHz can be obtained.
For practical application of an oscillator element operating on such a principle, the element needs to achieve both of a high Q factor of 100 or higher and a high oscillation output in the order of microwatts.
In order to achieve these, NPL 1 employs a point contact structure in which a nanoscale electrode is formed directly on a magnetoresistive thin film. This point contact structure is formed without etching of a magnetization free layer, and thereby is characterized in that: the structure has no shape magnetic anisotropy in the plane; the magnetization free layer is not physically or chemically damaged; and so forth. By employing this structure, a Q factor of approximately 18000 is obtained at maximum, and also a stable oscillation state having a Q factor at a level comparable with an oscillator circuit utilizing a quartz oscillator is obtained.
However, a GMR element is used as a magnetoresistive element in NPL 1. The GMR element has a low MR ratio, and is known to have difficulty obtaining a high oscillation output in the order of microwatts in principle.
To solve this problem, utilization of a TMR element having a high MR ratio is proposed as in NPL 2. However, both of a high Q factor and a high oscillation output have not been obtained yet.